Image sensors receive light and convert the energy into electrical signals based on the amount of light received. Image sensors generally include a silicon light-sensitive layer and electrical circuitry, such as an integrated chip, for processing the electrical signals. It is desirable to increase the pixel density on a chip to improve resolution. However, increasing pixel density creates issues such as crosstalk and denser circuitry wiring.
In frontside-illuminated (FSI) sensors, light enters the “front” of the chip where the metal interconnections lie. Increasing the amount of wiring causes more shadowing and therefore less efficiency of light reaching the silicon light-sensitive layer. In backside-illuminated (BSI) sensors, light enters the silicon side of the chip and therefore does not require passing through the metal interconnect layers. Thus, BSI sensors are desirable for advance image sensing technology. However, BSI sensors require thinner silicon in order for the light to pass through and reach the photoactive layer and therefore have not been as cost-effective to manufacture as FSI sensors. BSI sensors also require precision uniformity in thickness across the chip, which can be difficult to accomplish.
One approach for manufacturing thin semiconductor wafers is utilizing epitaxial silicon, in which layers of silicon are grown on a bulk silicon substrate. The silicon substrate is a very thin sheet of typically less than 1.5 microns. Another approach is using a thicker silicon wafer and grinding it down to the desired thinness after the circuitry and interconnect layers have been formed. Because it is difficult to achieve the necessary precision uniformity in thickness with grinding, BSI processing often requires both wafer level grinding and die level polishing. This creates throughput issues, increases cost, and can cause contamination. Other factors in fabricating BSI sensors include differences in processing temperatures at the various stages of manufacturing, which greatly affects the materials and sequence of manufacturing steps, and the ability to handle thin films during fabrication.